It’s been nine months since partition pruning code has been pushed into MySQL 5.1. Another available optimization, partition selection, has been in the main tree for even longer. There haven’t been any bugs reported for some time, so the code should be reasonably stable now.

And if you’re interested in what’s under the hood, a rather detailed description of partitioning optimizations is now available here:

It is a part of the internals manual, but I tried to write it so it doesn’t
require any knowledge of MySQL source code. (and don’t be scared away by my English - the text in the manual has passed the scrutiny of the documentation team

Hi! My name is Sergey Petrunia and I’m one of the developers in MySQL Query Optimizer team. If you don’t have a habit of leafing through the commits list, you probably don’t hear much about what we’re doing, and we don’t have your feedback on it.
This blog is my attempt to rectify this situation a bit. Let’s see if it will succeed.